A known on-board evaporative emission control system includes a vapor collection canister that collects fuel vapor emitted from a tank containing a volatile liquid fuel for the engine. During engine operation, vacuum from the engine intake manifold induces atmospheric air flow through the canister to desorb the collected fuel vapor, and draws the fuel vapor into the engine intake manifold for comsumption in the combustion process. A canister purge solenoid valve is under the control of a purge control signal generated by a microprocessor-based engine management system, and periodically purges the collected vapor to the engine intake manifold.
As the vapor collection canister collects fuel vapor, the canister gradually becomes saturated with the fuel vapor. It is believed that there is a need for a method and apparatus for determining the degree of saturation of the canister.